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Factor VIII (Antihemophilia Factor A)

Does this test have other names?

What is this test?

This test measures the activity of factor VIII, a blood-clotting protein. The test can find out whether you have hemophilia A or another clotting disorder.

Hemophilia is a rare inherited blood disorder. In this condition, blood fails to clot normally, which puts a person at risk of uncontrolled bleeding. Under normal circumstances, certain proteins, including factor VIII, come together to form blood clots and quickly stop bleeding.

If you are low on factor VIII proteins, you may have hemophilia A. Almost all people with hemophilia A are male. Women are generally only carriers of the gene and have a 50 percent chance of passing it on to each of their children. But it's possible for females to develop the disorder.

Because blood-clotting proteins work together to stop bleeding, the test may be done as part of an overall screening for the proteins involved in clotting.

Why do I need this test?

You may have this test if you have a family history of the disorder or if you have one or more of these symptoms:

Prolonged and unexplained bleeding after minor cuts or dental procedures

Easy bruising

Slow wound healing because of repeated bleeding or infection

Joint pain and stiffness

Swollen, hot joints or deformities

Heavy menstruation

Severe low back pain, usually on one side

The need for a blood transfusion after a minor injury

You may also have this test if you have abnormal results from other blood tests that measure your blood's ability to clot properly.

What other tests might I have along with this test?

Tests called partial thromboplastin time (PTT) and prothrombin time (PT) are usually the first step in hemophilia testing. These tests focus on clotting pathways. If you haven't already had these tests, you may have them along with your factor VIII test. Even if you have hemophilia, the results of your PTT and PT tests may be normal, so the blood test for factor VIII is used to confirm the diagnosis.

In addition, you will probably have your platelet count measured, which is part of a routine test called a complete blood count.

You may also have other procedures, including the factor VIII antigen assay. This is a separate test to determine the actual amount of factor VIII in your blood, not its clotting activity. Your doctor may also order a mixing study and a factor VIII inhibitor test, which look for antibodies in the blood that could deactivate factor VIII.

You may also have a von Willebrand factor test. The von Willebrand factor is a protein that "glues" platelets together to help form a clot. It protects factor VIII from breaking down.

If you are female and have a family history of hemophilia, your doctor may order molecular genetic testing to determine whether you are a carrier.

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

Test results are usually reported as a percentage of a "normal" result of 100 percent. Normal ranges for factor VIII levels are 50 to 150 percent.

If your factor VIII activity level is less than 50 percent, you may have hemophilia A, but how severe your risk of bleeding depends on what percentage you have.

If you have normal to decreased level of factor VIII, you may have von Willebrand disease.

Your doctor will look at your factor VIII test result along with the results of other tests to better understand what the results mean.

How is this test done?

The test requires a blood sample, which is drawn through a needle from a vein in your arm.

Does this test pose any risks?

Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

How do I get ready for this test?

You may need to stop taking certain medications before having the test. These may include NSAIDs like aspirin, ibuprofen (Motrin and Advil), or naproxen (Aleve). Be sure your doctor knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illicit drugs you may use.